At present, in a sheet cassette inserted in an image forming apparatus, a structure including a tray for stacking a sheet bundle and a pair of side walls to regulate the position of the sheet bundle in the width direction is mainstream. The sheet cassette further includes a coupling member (shaft+pin). When the sheet cassette is inserted in the inside of the image forming apparatus, the coupling member is inserted in a drive unit (coupling) fixed in the image forming apparatus and is coupled. The drive unit raises the tray to a position where the uppermost sheet of the sheet bundle stacked on the tray contacts a paper feed roller.
On the other hand, when the sheet cassette is pulled out from the inside of the image forming apparatus, there is a possibility that the uppermost sheet of the sheet bundle is placed in the state in which the sheet contacts the paper feed roller or is pinched. In this case, a friction force is exerted on the uppermost sheet by the paper feed roller. While the sheet cassette is pulled out from the inside of the image forming apparatus, the uppermost sheet is forced to remain in the inside of the image forming apparatus. There is a possibility that the uppermost sheet climbs over the side wall, and drops into a portion in the inside of the image forming apparatus and between the sheet cassette and the drive unit. Here, the portion in the inside of the image forming apparatus and between the sheet cassette and the drive unit is defined as a rear side space of the sheet cassette. Even if the uppermost sheet does not contact the paper feed roller, there is a possibility that the sheet drops into the rear side space of the sheet cassette by the shock when the sheet cassette is inserted into the inside of the image forming apparatus. This phenomenon is liable to occur when a maximum number of sheets are stacked on the sheet cassette.
When the sheet cassette is inserted into the inside of the image forming apparatus in the state where the sheet drops in the rear side space of the image forming apparatus, there is a possibility that various defects occur in the image forming apparatus. The sheet dropped in the rear side space of the sheet cassette cuts off the coupling between the coupling member and the drive unit, or damages these parts.
Although the defects as stated above exist, the user can not recognize that the sheet is significantly shifted from a normal set position in the sheet cassette, and further, the user can not recognize even that the sheet drops in the rear side space of the sheet cassette.